During nervous system development, growing axons navigate to their targets by responding to diverse cell-surface and secreted molecules. These "guidance molecules" act through multiple kinds of receptors to exert both stimulatory and inhibitory effects on axonal growth cones. The molecular mechanisms underlying these responses are poorly understood, yet insights into those mechanisms are likely to be of great importance in understanding and treating developmental disorders of the nervous system, as well as in improving the regenerative response to nerve, brain and spinal cord injury. Recently, we identified a protein, the B oligomer of pertussis toxin (PTb), that binds to chick primary sensory neurons and inhibits their responses to the guiding effects of three unrelated molecules--the extracellular matrix protein laminin (which can be patterned into pathways that growth cones closely follow), the secreted protein collapsin-1 (which repels growth cones and, under appropriate conditions, causes growth cone collapse), and the protease thrombin (which, acting through the thrombin receptor, causes growth cones to collapse and retract). Unlike intact pertussis toxin, PTb does not modify and inactivate G-proteins, rather, PTb acts by binding sialic acid- containing oligosaccharides on a subset of neuronal glycoproteins, and its effects on neurons can be mimicked by a sialic acid-binding plant lectin. It is unlikely that PTb acts by binding to and blocking the receptors for laminin, collapsin and thrombin, since PTb usually binds only a very small number of cell surface proteins with high affinity. In addition, PTb blocks only the guiding, and not the outgrowth-promoting effects of laminin. We hypothesize that PTb binds a cell surface receptor and modulates some signaling pathway in a way that specifically disrupts guidance by multiple classes of molecules. Elucidating such a signalling pathway could provide novel insights into how guidance molecules work and how their effects are integrated. We therefore propose to (1) investigate how PTb acts by testing specific hypotheses about the receptor mechanisms and signalling pathways involved in the inhibition of guidance responses by chick sensory neurons in tissue culture; (2) purify and identify the receptor through which PTb acts; and (3) look for direct associations of that receptor with known signaling pathways.